1. Field of the Invention
The present invention relates to an insulated wire, and in particular to a flame-retardant insulated wire which does not contain a halogen compound.
2. Description of the Related Art
Insulated wires provided with flame retardance are widely used in order to prevent fire from easily spreading to insulating coatings of the wires even in the case where flames are caused by abnormalities (e.g., generation of heat and tracking phenomena) at electric connection portions, such as wire joints and electric outlets. Among flame-retardant insulated wires, insulated wires not containing a halogen compound (non-halogen flame-retardant insulated wires) do not generate corrosive halogen gas (e.g., hydrogen chloride) or toxic gas (e.g., dioxin) even when they are burnt, and thus are advantageous in that the burden on the environment and health hazards can be reduced.
Examples of a known resin composition constituting the insulating coating of a non-halogen flame-retardant insulated wire include a resin composition in which a non-halogen flame retardant (e.g., a metal hydroxide, such as magnesium hydroxide) is added and mixed to a polyolefin resin. For example, Japanese Unexamined Patent Application Publication No. 2003-132741 (Patent Document 1) discloses an insulated wire including a conductor, an inner layer covering the conductor, and an outer layer covering the inner layer, in which the inner layer includes a flame-retardant resin composition including a base polymer containing 40 to 90 parts by weight of an ethylene copolymer, 5 to 50 parts by weight of an ethylene-acrylic rubber, and 0.5 to 50 parts by weight of an ionomer or ethylene-methacrylic acid copolymer, and also including 40 to 200 parts by weight of magnesium hydroxide and 1 to 20 parts by weight of a flame retardant promoter relative to 100 parts by weight of the base polymer; and the outer layer includes a resin composition including 100 parts by weight of an ionomer or ethylene-methacrylic acid copolymer, 300 parts by weight or less of magnesium hydroxide, and 20 parts by weight or less of a flame retardant promoter. According to Patent Document 1, after the conductor is covered with the inner layer includes the resin composition having excellent flame retardance and flexibility, the inner layer is covered with the outer layer having excellent abrasion resistance, and therefore, it is possible to provide an insulated wire having excellent abrasion resistance, flame retardance, cold resistance, and mechanical properties.
Furthermore, Japanese Unexamined Patent Application Publication No. 2010-97881 (Patent Document 2) discloses an insulated wire including a conductor, an inner layer covering the conductor, and an outer layer covering the inner layer, in which the inner layer contains an ethylene-ethyl acrylate copolymer (EEA) with an ethyl acrylate content (EA amount) of 10% to 20% by weight and has an insulating property; and the outer layer contains an ethylene-vinyl acetate copolymer (EVA) with a vinyl acetate content (VA amount) of 40% to 50% by weight and a non-halogen flame retardant, is crosslinked, and has oil resistance and flame retardance. According to Patent Document 2, it is possible to provide an insulated wire which does not contain a halogen compound and which has high flame retardance, high mechanical properties, a sufficient insulating property, and high oil resistance.
In recent years, regarding requirements for insulated wires, as the number of kinds of properties required has increased, the required levels of safety, durability, environmental protection, and the like have also increased. For example, in the case of wires for vehicle use, in addition to existing requirements for flame retardance, mechanical properties, and a halogen-free property, excellence in terms of oil resistance, fuel resistance, and cold resistance is also strongly required.
In the insulated wire described in Patent Document 1, in order to obtain high flame retardance, a metal hydroxide is added to each of the inner layer and the outer layer. When the addition amount of the metal hydroxide is increased in accordance with the required flame retardance level, mechanical properties (e.g., elongation of the insulating coating) may be degraded, which is a drawback. Furthermore, in the insulated wire described in Patent Document 2, the latest required levels of oil resistance, fuel resistance, and cold resistance are not necessarily satisfied. In other words, further improvements in the properties of insulated wires (in particular, non-halogen flame-retardant insulated wires) are strongly desired.